Expanding the Y Dwarf Census with Spitzer Follow-up of the Coldest CatWISE Solar Neighborhood Discoveries.

Abstract

We present Spitzer 3.6$\\mu$m and 4.5$\\mu$m follow-up of 170 candidate extremely cool brown dwarfs newly discovered via the combination of WISE and NEOWISE imaging at 3$-$5$\\mu$m. CatWISE, a joint analysis of archival WISE and NEOWISE data, has improved upon the motion measurements of AllWISE by leveraging a $>$10$\\times$ time baseline enhancement, from 0.5 years (AllWISE) to 6.5 years (CatWISE). As a result, CatWISE motion selection has yielded a large sample of previously unrecognized brown dwarf candidates, many of which have archival detections exclusively in the WISE 4.6$\\mu$m (W2) channel, suggesting that they could be both exceptionally cold and nearby. Where these objects go undetected in WISE W1 (3.4$\\mu$m), Spitzer can provide critically informative detections at 3.6$\\mu$m. Of our motion-confirmed discoveries, seventeen have a best-fit Spitzer [3.6]$-$[4.5] color most consistent with spectral type Y. CWISEP J144606.62$-$231717.8 ($\\mu \\approx 1.3 $/yr) is likely the reddest, and therefore potentially coldest, member of our sample with a very uncertain [3.6]$-$[4.5] color of 3.71 $\\pm$ 0.44 magnitudes. We also highlight our highest proper motion discovery, WISEA J153429.75$-$104303.3, with $\\mu \\approx 2.7 $/yr. Given that the prior list of confirmed and presumed Y dwarfs consists of just 27 objects, the Spitzer follow-up presented in this work has substantially expanded the sample of identified Y dwarfs. Our new discoveries thus represent significant progress toward understanding the bottom of the substellar mass function, investigating the diversity of the Y dwarf population, and selecting optimal brown dwarf targets for JWST spectroscopy.